Power inverters have long been used to convert DC power into AC power for energizing one or more AC loads. Typically, such inverters include switches such as transistors which are operated by a control in a pulse width modulated (PWM) mode to produce a PWM waveform comprising a series of pulses and notches. The waveform is converted into a sinusoidal output waveform by a filter which is coupled to the inverter output. Such an inverter may be used as part of a variable speed constant frequency (VSCF) power generating system in which a generator is driven by a variable speed prime mover to produce variable frequency AC power which is rectified, filtered and applied to the inverter as DC power over a DC link.
Ideally, the control should operate the inverter switches so that no DC power is produced in the output. However, operating conditions may cause DC content to be produced in the output of the inverter. This DC content can lead to undesirable consequences when the loads supplied by the inverter cannot tolerate same.
Sato U.S. Pat. No. 4,729,082 discloses a control for a power converter which converts between AC and DC power in a bidirectional manner. In order to eliminate the DC component of current on the AC side of the power converter, the control operates the converter to produce a DC voltage on the AC side which opposes the direct current component. The DC voltage is produced in one embodiment by shifting a half-cycle of the AC output waveform by a phase displacement which causes a DC component to be produced in the phase output. There is no clear description in this patent, however, as to how or by what means this is accomplished.